Machines for packaging pourable food products, such as fruit juice, wine, tomato sauce, pasteurized or long-storage (UHT) milk, etc., are known in which packages are formed from a continuous tube of packaging material defined by a longitudinally sealed strip.
The packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. In the case of aseptic packages for long-storage products such as UHT milk, the packaging material comprises a layer of barrier material defined, for example, by an aluminium film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material which eventually defines the inner face of the package contacting the food product.
For producing aseptic packages, the strip of packaging material is unwound off a reel and fed through a sterilizing unit in which it is sterilized, for example, by immersion in a bath of liquid sterilizing agent such as a concentrated solution of hydrogen peroxide and water.
More specifically, the sterilizing unit comprises a bath filled, in use, with the sterilizing agent in which the strip is fed continuously; and an aseptic chamber in which the strip of packaging material issuing from the sterilizing bath is treated to remove any residual sterilizing agent, e.g. by means of drying rollers and jets of high-temperature sterile air.
Before leaving the aseptic chamber, the strip is folded into a cylinder and sealed longitudinally to form in known manner a continuous, vertical, longitudinally sealed tube. The tube of packaging material, in fact, forms an extension of the aseptic chamber and is filled continuously with the pourable product and then fed to a forming and (transverse) sealing unit for forming individual packages and by which the tube is gripped between pairs of jaws to seal the tube transversely and form aseptic pillow packs.
The pillow packs are separated by cutting the sealed portions between the packs, and are then fed to a final folding station where they are folded mechanically into the finished form.
On known machines of the type briefly described above, the tube of packaging material is filled continuously by a conduit forming part of a circuit for supplying the pourable food product for packaging (hereinafter referred to simply as the "product circuit"). The product circuit must be sterilized with aseptic air before the start of each processing cycle, and flushed with a liquid solution at the end of the work cycle; for which purpose, the product circuit must therefore be connected selectively to a flush solution supply circuit and a sterile-air supply circuit by means of a switching assembly for preventing mixing of the various fluids and any traces of the flush solution in the product due to imperfect sealing. For safety reasons and to ensure sterility of the product circuit, the product circuit is normally connected to the switching assembly by means of a service conduit fitted with a two-way, aseptic, e.g. steam-barrier, on-off valve.
Substantially two switching assembly design solutions are known.
A first consists simply in using a movable fitting connected permanently to the service conduit of the product circuit and having connecting means for selective connection to the flush solution and sterile-air supply circuits. Though straightforward and cheap, this solution has the drawback of switching manually from one work stage to the other, by physically disconnecting and reconnecting the movable fitting to the relative circuits.
A second known solution consists in using three three-way valves, each of which has a pair of gates connected to a respective pair of circuits, and a third gate connected to a drain conduit for draining off any leakage from the valve and so preventing undesired leakage caused by pressure on the seals of the other valves.
Though enabling automatic switching, this second solution is expensive and involves considerable load losses.